We recently initiated a project to develop and validate Blitecast (potato) and Tomcast (for carrot) for Wisconsin, based on remotely sensed and forecasted weather data. The goals of this work were to provide the benefits of disease forecasting to a broader grower/user group and to utilize forecasted weather data to better anticipate accumulation of risk values for enhanced, proactive disease management.

We are still in the process of validating the use of this style of disease forecasting for Wisconsin. However, the data is useful in visualizing statewide disease risk trends and general 72-hour outlooks for accumulation of risk. We appreciate your feedback on this work. You can contact me at: gevens@wisc.edu.

Need-to-know concepts to make the maps useful

Blitecast and DSVs: Blitecast forecasts the risk for late blight in potato (can be extrapolated for tomato) based on accumulation of favorable weather. Accumulation of 18 disease severity values (DSVs) is the threshold. (read more detailed information)

Tomcast and DSVs: Tomcast forecasts the risk for foliar diseases in tomato and carrot (has been extrapolated and/or modified for several other vegetable diseases) based on accumulation of favorable weather. (read more detailed information)

Each map page has calculations for 4 time periods, positioned as indicated in the diagram below.

Click on a carrot in the chart below to lead to detailed diagrams for that calculation/emergence date combination

Emergence date (and day of year)

Calculation date (most recent at top)

17 May (137)

24 May (144)

31 May (151)

7 Jun
(158)

14 Jun
(165)

21 Jun
(172)

13 Aug

9 Aug

6 Aug

2 Aug

29 Jul

26 Jul

23 Jul

19 Jul

15 Jul

12 Jul

9 Jul

5 Jul

2 Jul

28 Jun

25 Jun

21 Jun

18 Jun

14 Jun

11 Jun

Detailed descriptions

Blitecast and DSVs: Blitecast forecasts the risk for late blight in potato (can be extrapolated for tomato) based on accumulation of favorable weather. Computation of 18 disease severity values (DSVs) relies on maximum and minimum temperatures each day, the duration of relative humidity periods above 90% and the maximum/minimum temperatures during the relative humidity periods above 90%. For a given day, up to 4 DSVs can accumulate. We start the severity value calculations at approximately 50% crop emergence. When we reach a total of 18 severity values, we issue a warning which indicates that environmental conditions have been met which favor late blight. At 18 DSVs, the recommendation for preventive applications of effective late blight fungicides is made. An additional alert is issued when the first symptoms of late blight appear anywhere in the state. The determination of late blight management recommendations is made by taking into consideration DSVs, projected weather forecast, and presence/risk of inoculum. This information is published in our newsletter and will be disseminated in various other outlets as the season progresses.

Tomcast and DSVs: Tomcast forecasts the risk for foliar diseases in tomato and carrot (has been extrapolated and/or modified for several other vegetable diseases) based on accumulation of favorable weather. Carrot Alternaria leaf blight and Cercospora leaf spot and Tomato early blight, Septoria, and anthracnose are the diseases targeted for control with use of Tomcast. DSVs accumulate on a daily basis (up to 4 per day) and are determined based on the number of hours of leaf wetness per day during specific temperature ranges (see table below).

Tomatoes transplanted to field before May 20 should have the first spray applied when DSV for that area exceed 25. In other Midwestern states, if no treatment has gone out on/before June 15, this date is targeted for the first spray. After the first spray, tomatoes are subsequently treated when the chosen spray interval (range 15-20 DSV) is exceeded. If transplant date is after May 20, tomatoes should be sprayed when they exceed chosen spray interval (range 15-20 DSVs) or by June 15.

Effective carrot fungicide programs typically begin at first symptom detection, with follow up applications timed at subsequent accumulations of 15-20 DSVs. Research in MI and WI has indicated effective control at 15 DSVs. For resistant or disease tolerant cultivars, a 20 DSV threshold may be effective.